1. Field of the Invention
The present invention relates to polymer dispersions which form elastic films of high blocking resistance and scratch resistance in the range of from 0 to 40.degree. C., to processes for their preparation, and to their use, for example, as binders for the coating of substrates.
2. Description of Related Art
To prepare aqueous coatings that are applied to substrates which are not dimensionally stable, (e.g., wood varnishes, stains and gloss paints), there is a need for binders with a level of elasticity sufficient to avoid cracking when the coating is exposed to weathering. In addition, however, these coatings are also required to have a certain hardness, so that the coating possesses good scratch resistance and blocking resistance. From an environmental perspective, it also is desired that the binder form a film within the range from 0 to 40.degree. C., so that little or no film consolidating agent is required.
From EP-B 0 184 091 and EP-B 0 015 644 it is known that it is possible, by multi-stage emulsion polymerization, to prepare aqueous auto-crosslinking polymer dispersions which have a low minimum film-forming temperature (MFT) and which form films with high blocking resistance. Owing to the use of a relatively high proportion of soft monomers in the first stage, however, the corresponding polymer films have deficiencies in the context of their scratch resistance. It generally was thought possible to compensate for this by using larger amounts of auto-crosslinking groups, but the result of such use would be a worsening in the elongation at break in addition to a higher price.
Multi-stage polymers as described in EP-B 0 332 011, possessing an MFT in the range from 65 to 110.degree. C., form highly scratch-resistant films but, when used as binders, require a large amount of film consolidating agent. Moreover, the elongation at break of the corresponding films made from these multi-stage polymers is not adequate for use on substrates which are not dimensionally stable.
Thus, there exists a need to provide polymers and polymer dispersions that can be made into films having sufficient elasticity and that have high blocking resistance and scratch resistance. There also exists a need to provide polymers and polymer dispersions that can be used as binders for coatings on substrates that are not dimensionally stable.